1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus for implanting shadowing agent to an examining object so as to examine a blood stream state, and its method.
2. Description of the Related Art
Various kinds of apparatus have been used as a medical application of ultrasound. Particularly, there has been mainly used an ultrasound diagnostic apparatus for obtaining a tomographic image of a soft tissue of a human body by an ultrasonic pulse reflection method. This kind of the ultrasound diagnostic apparatus displays the tomographic image of the tissue in a non-harmful examination method. This kind of the ultrasound diagnostic apparatus has the following features as compared with the other apparatus such as an X-ray diagnostic apparatus, an X-ray CT apparatus, an MRI apparatus (magnetic resonance image apparatus) and other medical diagnostic apparatus.
More specifically, a real time display can be performed; the apparatus is small-sized, and the manufacturing cost is low, and the safety of using the apparatus is high, i.e., no suffering from X ray.
Due to this, ultrasound diagnosis has been widely performed in the examination of the heart, the abdomen, the mammary gland, the urinary organs, and the obstetrics and gynecology. Particularly, the beat of the heart and the state of the movement of an embryo can be display at real time by the simple operation in which an ultrasonic probe is only touched on a surface of the body. Moreover, the examination is repeatedly performed due to high safety, and the examination when the apparatus is moved to the head side can be easily performed. Furthermore, the technique for image-forming the blood stream, which is called as a color-doppler method, is peculiar to the ultrasound diagnosis. However, in the color-doppler method, a relatively thick blood vessel can be image-formed, but the examination of a capillary cannot be performed.
In order to solve the above problem, attention has been recently paid to a contrast echo method using ultrasonic shadowing agent. In the contrast echo method, intensity of reflection from the blood stream is increased, thereby making it possible to clearly image-form the blood vessels including from a relatively thick trunk blood vessel to the capillary. As ultrasonic shadowing agent, minute bubbles have mainly used.
As diagnostic data obtained by the contrast echo method, there are a time series change of a spatial distribution of a shadow portion, time from when the shadowing agent is implanted till the agent reaches a reason region of interest (ROI), a time density curve (TDC) of luminance, and maximum luminance.
The following will explain the principle of the shadowing effect briefly.
The bubbles have extremely small acoustic impedance, and the difference between the acoustic impedance of the bubbles and that of the tissue is extremely large. Therefore, intensity of reflection at a boundary between the bubbles and the tissue is extremely larger than intensity of reflection at a boundary between the tissues, so that the shadowing effect is exerted.
However, the following problem is present in the case of the bubbles.
If the bubbles receive ultrasound, the bubbles disappear by its sound pressure. The bubbles in the blood expand like a spring by vibration of the sound pressure. If the bubbles expand and contract by a resonance frequency in accordance with its diameter, amplitude of expansion and contraction shows a maximum diameter, and a reduction speed of an amount of bubbles becomes maximum.
All bubbles do not necessarily disappear at one time of receiving and transmitting ultrasound. However, by the repetition of receiving and transmitting ultrasound several times per one second, shadow agent substantially disappears instantaneously. Due to this, the shadowing effect is reflected on only the image for several frames, and it is difficult to confirm the shadowing effect on a motion image for 30 frames per second if the amount of implanted shadowing agent is small.
In order to solve the above problem, output power (sound pressure) of ultrasound may be reduced to the extent that shadowing agent does not appear for a fixed period of time. However, the reduction of output power lowers spatial resolution. Also, in order to solve this problem, concentration of shadowing agent to be implanted may be increased, or shadowing agent may be repeatedly implanted. However, in consideration of danger to the examining human body, this method is unfavorable.
In order to extend the life of the shadowing agent, it is preferable to scan the examining human body with ultrasound of low power. In this case, however, the contrast is lowered. In order to obtain an image of a high contrast, on the other hand, it is necessary to scan the examining human body with ultrasound of a high power. In this case, however, the use of the high-power ultrasound will shorten the life of the shadowing agent. In order to make the life of the shadowing agent as long as possible and, at the same time, obtain a high contrast image, it may be considered that the examining human body be scanned, only once for a few seconds, with the high-power ultrasound. This method, however, poses the problem of a lowered time resolution.